Input impedance of the Ganglion board?

mark.o.s

I've checked the AD8237 datasheet and it mentions in differential mode Zin = 100MOhm || 5pF and in Common mode Zin = 800MOhm || 10 pF.

In the Ganglion, is the AD8237 always in differential mode?

 

wjcroft

Mentioning Joel @biomurph.

bpwork

Hi Mark.  First, about differential and common mode, they're not the kinds of mode where you would put the device into a certain mode.  Instead, it's a matter of which pins of the device are being driven; and often they're being driven partially in each mode at the same time.  Classically, 

-- Differential mode would mean, if you apply a voltage Vin between A) the + input, and the - input; and you look at how much current (Iin) flows in that voltage source; then Vin/Iin is the differential mode input impedance.  

-- Common mode would mean, if you apply a voltage Vin between A) the + input & the - input which are connected together, and the amplifier's ground; and you look at how much current (Iin) flows in that voltage source; then Vin/Iin is the common mode input impedance.

But in Ganglion, neither of those AD8237 specs matters much at all, because the input impedance of Ganglion is dominated by other components which have much lower impedance and are in parallel with the AD8237 inputs (and being in parallel, the lower impedances dominate).  The short story is that the input impedance of Ganglion, for the EEG/etc frequency range where Ganglion runs, will be about 660K Ohms.  Or a little less depending on some things (such as, actually, how much of the signal is common mode in addition to the predominant differential mode .... long story).  I wouldn't expect the overall input impedance to be greater than 670K or less than, say, 620K.

I hope that helps, I could explain more detail if you ask but I might not get to it too soon.  I think the best schematic to look at would be a nice new one that Joel put up here:

https://github.com/OpenBCI/Ganglion_Hardware_Design_Files/blob/master/Ganglion_SCH.pdf

Or for just a subset of the analog circuitry (channel-1-only) I re-drew it in a way that helps me intuit things better, I could find & point to it.

    -- Bruce P.

bpwork

Hmm, my pointer to the schematic on github doesn't seem to work for me.  Just goes to show how little I know about that sort of thing.  

Maybe you know how to find it if I say that it's on github, OpenBCI / Ganglion_Hardware_Design_Files / Ganglion_SCH.pdf.

wjcroft

Bruce, I fixed your link. The links in this Vanilla Forum software seem to work best on separate lines. If text follows the link, it sometimes screws up.

Just for reference, on Cyton the ADS1299 input impedance (according to TI data sheet), is 1 gigaohm. Or 500 megohms when in impedance measurement mode.